A zoom lens means a lens which is capable of successively changing its focal length while keeping a focal position of an entire lens system constant. Generally, in order to configure a zoom lens, it is necessary to relatively move at least two groups of lenses on an optical axis. That is, when only one group is relatively moved, in the optical system composed of the two groups of lenses, it is possible to change a combined focal length, meanwhile a focal position as well is changed. On the other hand, it is possible to change a combined focal length without changing a focal position by moving both of the two groups (refer to Non-Patent Document 1).